1. Field of the Invention
The present invention relates to a method and system for recording image data by use of a thermal head.
2. Description of the Related Art
Characteristics of a thermal recording apparatus, such as a thermal transfer recording apparatus or a heat sensitive recording apparatus include excellent consistency in the quality of resultant hard copy, low noise, and low cost. A thermal recording apparatus records image data by heating a thermal head. In the thermal transfer recording apparatus with a thermal head and an ink ribbon (ink film), the ink ribbon is placed between the thermal head and a recording paper on a platen roller. To record the image data, the thermal head is pressed against the recording paper, heating elements contained in the thermal head is heated, and ink of the ink ribbon is transferred onto the recording paper.
A facsimile apparatus generally employs an image recording system of the heat sensitive recording apparatus or the thermal transfer recording apparatus. In the image recording systems, after a recording paper is vertically shifted or fed, the image data of each line is horizontally recorded on the recording paper. In this case, a line type thermal head is used whose width corresponds to the width of the recording paper. In the thermal head, heating elements are linearly arrayed. The direction in which the array of heating elements extends when the thermal head is set to the recording apparatus, is called the main scan direction, while the direction orthogonal to the main scan direction, viz., the direction in which the recording paper is fed, is called the sub scan direction.
To short the facsimile data transmission time, the image data read is digitized for each picture element (pixel), and encoded before being transmitted. The widely used encoding processes are:
(1) To encode image data in accordance with the number of pixels of the same color running consecutively in the encoding line. The length of such consecutive pixels is called a run-length, more exactly, the length of consecutive white or black pixels extending in the scanning direction (one-dimensional encoding).
(2) To encode image data by determining modes depending on the states of pixels at the corresponding positions between an encoding line and a reference line previous to the encoding line (two-dimensional encoding).
In the encoding process (1) above, the pixels are reproduced on the basis of a variable run-length. In the encoding process (2), a mode is determined on the basis of a variable length code, and the image reproduction is performed by obtaining the position of changing of the image representing the pixel, for example, when the image changes from white to black. A speed of encoding and decoding varies depending on a state of an image, and hence the recording speed is nonuniform. In the recording operation of the heat sensitive type and of the thermal transfer type, a recording paper is fed, and the inertia causes the recording paper to possibly overrun in the feeding motion. The feeds of the paper are irregular. Because of this, space is produced between adjacent recording lines. To cope with the production of the space, a conventional recording is to use a thermal head having the heating elements which are longer in the sub scan direction than in the main scan direction, and to use a paper feed pitch which is slightly shorter than the length of the heating element in the sub scan direction. In an image recording, the images of the two adjacent recording lines partially overlap.
The lead wires coupled with each heating element are made of conductive material of excellent heat transmission property. Accordingly, the energy of heat of the heating element near the lead wires decreases faster than the energy of heat in the central part. Therefore, a recording energy distribution curve of the heating element of the conventional thermal head peaks at the center of the element. To heat the heating element so that a predetermined recording density can be obtained within a predetermined recording area, it is necessary to supply to the heating element the energy of heat in excess of a recording energy corresponding to a maximum recording density. Further, temperature at the central part of the heating element is higher than that at the surrounding part. Therefore, when the image is thermally transferred onto a recording paper, the ink at the central part is apt to flow toward the surrounding part, so that a recording density is reduced at the central part and hence a recording density is irregular over the recorded image within the pixel area.
A thermal response of the heating element is problematic when the image is recorded at a constant speed. In the conventional heating element, a cooling speed at the central part of the heating element is slow. Accordingly, a long time is taken till the temperature at the central part decreases to an unrecordable temperature. For this reason, when the color of the image as a pixel changes from black to white, an image signal indicating white is applied to the heating element, but black remains due to its heat inertia.
In the image recording of the thermal transfer type, a degree of contact of the ink film with the recording paper affects an image quality of the recorded image. Allowing for this, a thermal head of a partial graze type in which each heating element is protruded at the central part, has been employed. A peak pressure of the partial graze type thermal head is higher than that of an overall graze type thermal head. Therefore, the partial graze type thermal head can make a closer contact of the ink film and the recording paper than an overall graze type thermal head whose heating element has a flat top. In this respect, the former is superior to the latter. However, in the partial graze type thermal head, the ink at the central part of the heating element tends to flow toward its surrounding part, so that a recording density at the central part is lower than that at the surrounding part.
The thermal head is fixed at one end, and free at the other end, so that it is swingable about the fixed end, which acts as a fulcrum. Such a structure is employed to ensure the positioning of the recording paper and the ink film in the thermal recording apparatus. If a backlash in the moving structure occurs, the center of the heating element shifts to another position. If the center is so shifted, the heating element is partially out of a recordable area. A part of the heating element out of the recordable area fails to press the ink film against the recording paper, so that an image recorded under such a condition is imperfect while having non image part corresponding to the press-failed-part. This is proper to the partial graze type thermal head, and exists in both the thermal transfer type recording apparatus and the heat sensitive type recording apparatus. A contact pressure between the thermal head and the platen roller is higher in the central part of the heating element than in the surrounding part. Therefore, a degree of contact therebetween is excellent. However, a high contact pressure expels the melted ink from the central part of the heating element toward the surrounding part. As a result, a recording density at the central part is reduced. In addition, a thickness of the ink layer over the recording paper is nonuniform. This brings an instable peeling-off of ink. An image recorded under such a condition blurs in its contour.
For the above background reasons, there is a desire for image recording apparatuses which is low in power consumption and free from reduction of a recording density and deterioration of an image quality.